1. Field of the Invention
The present invention relates generally to improvements in web manufacturing machinery and, more particularly, but not by way of limitation, to improvements in local web profile control using a flexible shaping member positioned to engage one side of the web and flexed by thermal expansion rods to profile such side of the web.
2. Brief Description of the Prior Art
In the manufacture of webs of plastic material, it is often necessary to provide dynamic control of the thickness of the web across its width. Where the web is formed from a fluid material, this control can be accomplished by providing the machine that manufactures the web with an elongated shaping member that engages the web material at a location in the machine at which the material is in a plastic state and flexing the shaping member along its length, which is transverse to the direction of movement of the web material, to profile one side of the material in a way that will cause the completed web to have a constant thickness throughout. For example, where the web is being produced by forcing molten plastic between the lips of an extrusion die, the shaping member is one of the lips of the die. By appropriately flexing such lip, the web thickness is maintained constant despite uneven flow of molten material to various portions of the die lips. Similarly, where the web is being produced by coating a previously formed web with a fluid component, a flexible doctor knife can be provided to adjust the thickness of the fluid component along the length of a coating roller by means of which the coating is applied to the substrate. In this case, the doctor knife is the flexible shaping member.
The flexing of the shaping member is effected by a series of force exerting devices that are arranged in a line down the length of the shaping member and the forces these devices exert on the shaping member can be controlled by a computer that receives signals from one or more web thickness sensors positioned to determine the thickness of a completed web. For example, where the web is manufactured by extrusion, a beta gauge might be positioned on the path the web follows after leaving the extrusion die and the gauge might be scanned back and forth across the width of the web to provide signals indicative of the web thickness across its width to the computer. The computer can then be programmed to actuate the force exerting devices in a manner that will keep the web thickness within preselected tolerances throughout the web width.
A particularly useful type of force exerting device for flexing the shaping member is a thermal expansion rod; that is, a rod which is equipped with a heating element so that portions of the rod can be caused to expand by varying amounts to adjust the force the rod exerts on objects engaged by the rod. One end of the rod is fixed and the other end bears against the shaping member so that the flexing of the shaping member can be controlled by the rate at which electrical energy is supplied to the heating elements on the expansion rods. Since the power supplied to each of the heating elements can be precisely controlled, the thermal expansion rods offer a capability for precise control of the shape of the shaping member and this control, in turn, offers a possibility for precise control of the thickness of the web across its width.
However, the full potential of web thickness control systems using a line of thermal expansion rods to position portions of a shaping member in contact with material of which a web is being formed has not been fully realized because of interacting practical problems that arise in the implementation of these systems. Initially, the effect of one thermal expansion rod on the shaping member cannot be treated independently of the effect of all of the other rods on the shaping member. Rather, the shaping member and the thermal expansion rods form an elastic system which must be considered as a whole in programming a computer to vary the currents through the heating elements on the thermal expansion rods to change the shape of the shaping member in a way that is dictated by the measured thickness of various portions of the web being produced. In particular, to cause the shaping member to have a particular configuration, the computer must be programmed to cause the heating of the totality of rods to result in some rods pushing on the shaping member while other rods pull on the shaping member. Moreover, any particular rod may, at times, be under compression and, at other times, be under tension to maintain the thickness of the web at a preselected value across the entire web. The programming itself presents no real problems because the thermal expansion coefficients of the rods and the elastic properties of the rods and shaping member are known so that it is possible to develop an accurate mathematical model of the system. The difficulty lies in the need for both pushing and pulling on the shaping member in conjunction with a second characteristic of the control system.
The second characteristic is a limitation in the range of web thickness variation that can be effected using thermal expansion rods. The lengths of the rods are subject to practical limitations so that, in general, it is not possible to make the rods long enough that any desired control range for the web thickness can be achieved solely by the heating of the rods. Rather, a control range must be selected and the fixed ends of the rods must be positioned so that heating of the rods can be used to maintain the web thickness within the selected control range. This need to be able to position the fixed ends of the thermal expansion rods conflicts with the need for a capability of the rods to be placed in tension as well as in compression.
Any mechanism that might be used to fix one end of a thermal expansion rod so that the other end of the rod can push or pull on a shaping member in a web manufacturing machine, as the occassion demands, must satisfy three practical requirements. It must be free of backlash; it must be inexpensive to manufacture; and it must be reasonably easy to adjust. The latter two requirements stem from the economics of web manufacture. In general, webs which are suited to manufacture using a flexible shaping member are manufactured in large widths and cut to size so that many thermal expansion rods may be needed to position its portions. The sheer number of the mechanisms required to fix one end of each end of these rods thus imposes the latter two conditions. As to the remaining condition, backlash would present an anomaly in the model used to develop a computer control program so that it would not be possible for the computer to predict the effect of adjusting the amount of power delivered to the heating elements on the thermal expansion rods. Rather, where backlash exists, it would be necessary to adopt a scheme of adjusting the power delivered to the heating elements, measuring the effect of the adjustment, and then making successive adjustments until the web thickness is within tolerances. Such a scheme is undesirable. By the time the adjustment of the power levels can be completed using such a scheme, the machine that manufactures the web may have produced a large quantity of nonstandard, and therefore commercially unacceptable, web.
The net result is that, while thermal expansion rod control of web manufacturing machines has been a useful technique, it has not been possible to realize the full potential of the technique. In general, prior art machines that make use of such control are designed to exert only unidirectional forces on the shaping member so that complete control of the shaping member has not been achieved in the past.